This invention relates to an emulsion ink of a water-in-oil (w/o) type useful for stencil printing and, more particularly, to a water-in-oil type emulsion ink for stencil printing which contains an esterified vegetable oil and which is environmentally friendly, safe and stable.
In stencil printing, an ink is applied onto a perforated stencil master backed by a printing paper and is passed through the perforations to form an image on the printing paper. A well-known ink for use in stencil printing is a water-in-oil type (w/o) emulsion ink containing a volatile solvent, a non-volatile solvent, a resin, a coloring agent, a surfactant, water, an antifreezing agent, an electrolyte and an antiseptic agent.
In recent years, an ink containing a vegetable oil is used as an ink for stencil printing which is safe and stable in any environment and can make good prints having a high image density and free from offset.
For example, JP-A-H10-245516 discloses a w/o type emulsion ink for stencil printing containing an vegetable oil having an iodine value of 100 or lower and a melting point of 0° C. or lower. In this ink, a non-volatile oil having an iodine value of 100 or lower is used to prevent solidification of the ink. However, water in the ink vaporizes and the viscosity of the ink decreases with time. As a result, an excess amount of the ink is loaded onto a paper to cause offset.
JP-A-H10-259341 discloses a w/o type emulsion ink for stencil printing containing an animal fat or vegetable oil ester comprising a fatty acid, and an oil or fat having a melting point of 20–55° C. Thus, when the ink is used after having been maintained in a low-temperature environment, good prints cannot be made.
JP-A-2002-220560 discloses a w/o type emulsion ink for stencil printing containing a vegetable oil having an iodine value of 110–150. This ink contains a vegetable oil having an iodine value of 110–150 and an oxidation inhibitor so that the ink does not cause clogging of the screen or offset even when left unused for a long time. The oxidation inhibitor is dibutylhydroxytoluene, propyl gallate, butylhydroxyanisole or the like. It is well known that these oxidation inhibitors are stable but not safe. Thus, an oxidation inhibitor which is safer than the conventional oxidation inhibitors is demanded. However, under the current circumstances, it is difficult to obtain an oxidation inhibitor excellent in both stability and safety. In addition, when an oxidation inhibitor is used, the vegetable oil undergoes oxidation when the effect of the oxidation inhibitor is lost.
Oils used in making foods and beverages (waste edible oils) are recycled and reused in the production of animal feed, soap and other products or as fuel. However, waste edible oils are increasing every year and cannot be used up.